Hewlett Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hewlett Observatory clear sky chart from 2005-11-06 to 2024-01-31.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
25%
22729
12%
75003
7%
22687
31%
22687
14%
55318

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
11%
760
6%
2649
2%
757
17%
757
5%
2229
January11th to 20thNight
13%
760
6%
2660
4%
760
16%
760
6%
2235
January21st to endNight
17%
835
9%
2844
4%
835
23%
835
9%
2245
February1st to 10thNight
10%
720
6%
2340
1%
720
16%
720
6%
1936
February11th to 20thNight
13%
703
11%
2230
4%
700
22%
700
10%
1736
February21st to endNight
16%
592
12%
1776
5%
592
24%
592
11%
1386
March1st to 10thNight
19%
720
13%
2160
8%
720
26%
720
12%
1671
March11th to 20thNight
17%
720
13%
2007
4%
720
25%
720
14%
1532
March21st to endNight
22%
792
19%
2043
7%
792
29%
792
18%
1516
April1st to 10thNight
19%
720
15%
1800
4%
720
24%
720
17%
1364
April11th to 20thNight
23%
711
15%
1775
4%
711
28%
711
19%
1321
April21st to endNight
23%
662
14%
1742
6%
662
28%
662
18%
1302
May1st to 10thNight
30%
360
14%
1440
11%
360
39%
360
15%
963
May11th to 20thNight
29%
354
19%
1416
12%
354
44%
354
18%
881
May21st to endNight
36%
388
13%
1551
12%
388
45%
388
20%
938
June1st to 10thNight
38%
360
11%
1440
11%
360
46%
360
16%
975
June11th to 20thNight
35%
360
12%
1440
15%
360
43%
360
18%
886
June21st to endNight
35%
360
10%
1440
12%
360
42%
360
16%
960
July1st to 10thNight
45%
350
14%
1400
18%
350
53%
350
24%
796
July11th to 20thNight
45%
348
16%
1392
18%
348
54%
348
24%
825
July21st to endNight
42%
394
14%
1486
13%
372
48%
372
22%
897
August1st to 10thNight
47%
385
15%
1449
13%
383
53%
383
21%
892
August11th to 20thNight
49%
583
16%
1647
11%
583
52%
583
27%
1108
August21st to endNight
41%
792
16%
1980
8%
792
44%
792
30%
1338
September1st to 10thNight
41%
720
15%
1800
6%
720
44%
720
30%
1226
September11th to 20thNight
49%
720
21%
1820
13%
720
53%
720
33%
1176
September21st to endNight
40%
720
20%
2001
15%
720
44%
720
25%
1348
October1st to 10thNight
30%
720
14%
2160
11%
720
34%
720
18%
1536
October11th to 20thNight
26%
720
13%
2160
7%
720
34%
720
16%
1559
October21st to endNight
19%
792
10%
2529
5%
792
26%
792
12%
1965
November1st to 10thNight
23%
740
14%
2407
8%
740
31%
740
16%
1706
November11th to 20thNight
22%
760
14%
2588
10%
760
29%
760
13%
1872
November21st to endNight
17%
760
9%
2660
5%
760
23%
760
8%
2102
December1st to 10thNight
12%
760
5%
2660
2%
760
15%
760
5%
2273
December11th to 20thNight
14%
760
9%
3256
5%
759
20%
759
6%
2586
December21st to endNight
11%
828
3%
4855
2%
817
15%
817
3%
4037

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
19%10%47%6%3445%6%31%
74888
Night
20%12%47%6%34446%30%
75235
TransparencyDay
54%6%12%19%9%1
74593
Night
56%5%12%17%8%1
75064
SeeingDay
27%15%29%19%8%2
13491
Night
27%14%28%23%8%1
22738

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
21%19%6%11%7%45%33417%
1517
11th to 20thDay
20%18%6%11%8%3434418%
1520
21st to endDay
14%16%7%11%10%5%5%44420%
1734
February1st to 10thDay
19%16%6%13%7%4444420%
1620
11th to 20thDay
17%16%6%10%8%345%45%23%
1617
21st to endDay
21%16%7%10%8%4434420%
1480
March1st to 10thDay
16%15%7%11%7%3443427%
1797
11th to 20thDay
21%13%5%8%6%3433429%
1951
21st to endDay
22%12%46%5%23336%34%
2312
April1st to 10thDay
22%12%46%5%34345%32%
2160
11th to 20thDay
23%9%345%34335%37%
2134
21st to endDay
26%9%36%5%23446%32%
2233
May1st to 10thDay
21%8%36%5%345%5%6%35%
2520
11th to 20thDay
18%7%35%5%35%447%40%
2478
21st to endDay
16%7%346%35%5%6%9%36%
2725
June1st to 10thDay
23%7%25%5%345%6%7%31%
2520
11th to 20thDay
19%5%35%5%35%46%8%36%
2520
21st to endDay
21%6%35%5%35%5%5%7%34%
2520
July1st to 10thDay
14%5%34435%5%7%8%42%
2520
11th to 20thDay
15%5%36%6%35%5%6%8%37%
2520
21st to endDay
14%5%25%5%35%6%8%9%38%
2744
August1st to 10thDay
14%5%26%5%45%5%6%8%38%
2476
11th to 20thDay
13%6%36%5%36%5%7%8%37%
2236
21st to endDay
16%5%25%6%35%5%5%8%38%
2376
September1st to 10thDay
17%6%25%5%345%5%7%42%
2160
11th to 20thDay
16%6%34433447%45%
2139
21st to endDay
19%6%345%35%6%5%6%38%
1958
October1st to 10thDay
24%7%36%6%4445%6%32%
1800
11th to 20thDay
21%12%47%6%35%5%5%6%26%
1800
21st to endDay
29%10%36%5%44346%25%
1825
November1st to 10thDay
18%8%46%7%45%5%6%7%30%
1671
11th to 20thDay
19%12%47%6%5%445%5%29%
1587
21st to endDay
21%17%5%9%7%34445%20%
1520
December1st to 10thDay
26%21%6%8%7%34335%13%
1520
11th to 20thDay
21%19%7%9%6%2443421%
1906
21st to endDay
27%18%5%10%6%4333417%
2772
January1st to 10thNight
20%20%7%11%8%3334417%
2649
11th to 20thNight
19%19%6%11%9%45%34417%
2660
21st to endNight
14%18%7%11%6%45%445%23%
2844
February1st to 10thNight
19%18%7%11%8%3445%417%
2340
11th to 20thNight
16%15%7%10%8%35%445%24%
2228
21st to endNight
19%15%7%9%7%44345%25%
1776
March1st to 10thNight
18%16%7%8%7%3433427%
2150
11th to 20thNight
21%15%47%5%3433430%
2006
21st to endNight
21%13%46%6%2333436%
2043
April1st to 10thNight
24%9%47%6%23445%32%
1800
11th to 20thNight
23%8%45%5%34445%35%
1775
21st to endNight
24%11%46%434445%33%
1742
May1st to 10thNight
24%8%35%5%245%5%8%32%
1440
11th to 20thNight
14%7%37%6%345%46%41%
1415
21st to endNight
15%6%35%5%35%45%8%42%
1550
June1st to 10thNight
21%5%36%6%35%5%5%8%32%
1440
11th to 20thNight
18%5%35%6%3445%8%40%
1440
21st to endNight
18%8%36%5%35%5%5%8%34%
1440
July1st to 10thNight
12%5%25%5%35%46%6%46%
1440
11th to 20thNight
12%5%35%5%35%6%6%10%40%
1440
21st to endNight
12%6%26%435%5%7%9%41%
1574
August1st to 10thNight
13%6%15%6%46%5%6%8%40%
1459
11th to 20thNight
16%6%36%6%345%47%41%
1647
21st to endNight
13%6%35%6%25%45%7%43%
1980
September1st to 10thNight
16%6%35%5%2435%7%44%
1800
11th to 20thNight
16%5%34423447%48%
1820
21st to endNight
17%6%36%5%4445%7%39%
2001
October1st to 10thNight
23%8%35%6%345%5%6%34%
2160
11th to 20thNight
21%9%36%6%35%45%6%32%
2160
21st to endNight
28%10%37%6%44445%25%
2529
November1st to 10thNight
19%11%46%5%2445%6%33%
2407
11th to 20thNight
20%11%47%5%34446%30%
2588
21st to endNight
23%16%5%8%5%4444423%
2660
December1st to 10thNight
24%19%7%10%7%3444414%
2660
11th to 20thNight
24%17%5%8%6%3433423%
3288
21st to endNight
31%17%6%10%5%3433415%
4884

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
72%5%9%12%2
1518
11th to 20thDay
71%39%9%8%1
1520
21st to endDay
68%411%10%5%1
1734
February1st to 10thDay
70%48%14%31
1620
11th to 20thDay
65%411%12%7%
1626
21st to endDay
69%38%12%6%1
1480
March1st to 10thDay
63%310%14%8%1
1800
11th to 20thDay
60%410%13%11%2
1951
21st to endDay
54%37%18%13%4
2312
April1st to 10thDay
57%37%20%11%2
2160
11th to 20thDay
52%47%20%15%3
2134
21st to endDay
54%410%15%14%3
2233
May1st to 10thDay
51%5%9%20%14%2
2520
11th to 20thDay
45%5%10%24%14%2
2482
21st to endDay
45%7%14%21%12%2
2725
June1st to 10thDay
51%7%12%18%11%1
2520
11th to 20thDay
44%6%12%24%11%2
2520
21st to endDay
49%7%11%20%11%1
2520
July1st to 10thDay
38%7%14%27%14%1
2450
11th to 20thDay
42%8%16%26%8%
2445
21st to endDay
40%9%18%24%9%
2591
August1st to 10thDay
42%8%16%24%9%
2462
11th to 20thDay
43%8%18%22%9%
2236
21st to endDay
43%7%14%26%10%
2376
September1st to 10thDay
41%6%15%24%12%1
2160
11th to 20thDay
39%5%12%30%14%1
2139
21st to endDay
44%7%13%26%9%
1958
October1st to 10thDay
54%5%11%20%10%1
1800
11th to 20thDay
57%7%14%15%5%1
1800
21st to endDay
61%5%12%14%7%1
1825
November1st to 10thDay
53%6%14%20%6%1
1671
11th to 20thDay
59%5%12%16%7%2
1587
21st to endDay
67%6%12%10%4
1520
December1st to 10thDay
76%411%8%2
1520
11th to 20thDay
69%7%11%11%2
1906
21st to endDay
73%5%7%13%2
2772
January1st to 10thNight
73%5%9%10%3
2649
11th to 20thNight
73%48%9%5%
2660
21st to endNight
66%5%10%13%6%1
2844
February1st to 10thNight
71%5%10%10%4
2340
11th to 20thNight
64%410%17%5%
2230
21st to endNight
62%5%10%15%8%
1776
March1st to 10thNight
62%310%15%8%1
2160
11th to 20thNight
59%49%16%9%1
2007
21st to endNight
55%46%20%13%3
2043
April1st to 10thNight
56%48%20%11%1
1800
11th to 20thNight
52%6%9%18%14%2
1775
21st to endNight
55%49%17%12%3
1742
May1st to 10thNight
50%5%11%14%16%3
1440
11th to 20thNight
45%5%9%23%15%3
1416
21st to endNight
41%6%16%19%14%3
1551
June1st to 10thNight
49%7%14%18%10%1
1440
11th to 20thNight
43%6%14%20%16%1
1440
21st to endNight
47%8%14%19%12%
1440
July1st to 10thNight
35%7%14%25%17%2
1400
11th to 20thNight
38%7%14%29%12%
1392
21st to endNight
38%9%17%24%11%1
1486
August1st to 10thNight
40%8%19%22%11%
1449
11th to 20thNight
42%6%14%23%14%1
1647
21st to endNight
39%7%15%29%10%
1980
September1st to 10thNight
41%6%13%27%12%1
1800
11th to 20thNight
36%6%14%30%13%1
1820
21st to endNight
44%5%12%27%11%1
2001
October1st to 10thNight
51%6%13%20%10%1
2160
11th to 20thNight
53%6%15%18%7%1
2160
21st to endNight
61%5%14%12%7%1
2529
November1st to 10thNight
52%6%15%17%9%1
2407
11th to 20thNight
55%6%14%17%6%2
2588
21st to endNight
65%5%13%12%4
2660
December1st to 10thNight
75%5%10%9%1
2660
11th to 20thNight
67%5%9%14%4
3288
21st to endNight
73%6%9%11%1
4884

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
29%23%28%14%6%
180
11th to 20thDay
29%21%33%12%41
180
21st to endDay
27%21%30%19%3
228
February1st to 10thDay
42%28%25%5%
240
11th to 20thDay
32%29%30%8%
284
21st to endDay
37%21%28%12%2
346
March1st to 10thDay
30%19%31%15%4
420
11th to 20thDay
34%19%31%13%21
420
21st to endDay
36%18%25%13%7%1
462
April1st to 10thDay
35%16%31%13%4
420
11th to 20thDay
35%17%21%17%6%3
412
21st to endDay
37%15%28%13%7%1
420
May1st to 10thDay
29%17%27%18%7%1
423
11th to 20thDay
23%15%32%20%8%2
447
21st to endDay
21%14%31%23%11%1
485
June1st to 10thDay
29%14%28%21%6%2
450
11th to 20thDay
18%14%35%21%9%3
450
21st to endDay
25%13%31%18%11%2
450
July1st to 10thDay
18%9%31%28%13%2
445
11th to 20thDay
16%12%27%30%11%3
444
21st to endDay
18%7%35%31%8%2
488
August1st to 10thDay
18%8%25%30%16%3
446
11th to 20thDay
15%7%28%27%16%7%
439
21st to endDay
17%12%32%22%11%7%
495
September1st to 10thDay
18%11%26%25%13%7%
450
11th to 20thDay
16%8%28%23%15%10%
450
21st to endDay
20%14%26%24%14%3
450
October1st to 10thDay
29%16%23%15%13%5%
450
11th to 20thDay
30%18%28%18%6%2
450
21st to endDay
37%14%26%19%4
417
November1st to 10thDay
28%19%28%16%8%1
365
11th to 20thDay
33%16%32%11%6%2
247
21st to endDay
34%18%34%8%6%
180
December1st to 10thDay
41%14%26%15%31
180
11th to 20thDay
33%24%31%11%1
180
21st to endDay
40%17%27%14%21
198
January1st to 10thNight
33%18%29%17%3
760
11th to 20thNight
33%21%25%16%4
760
21st to endNight
24%19%31%20%5%
835
February1st to 10thNight
31%22%31%15%2
720
11th to 20thNight
26%19%36%16%3
703
21st to endNight
27%19%32%18%3
592
March1st to 10thNight
28%18%28%22%3
720
11th to 20thNight
30%19%30%16%41
720
21st to endNight
31%13%30%20%7%
792
April1st to 10thNight
30%16%32%20%3
720
11th to 20thNight
31%16%26%20%7%
711
21st to endNight
31%13%28%19%7%
662
May1st to 10thNight
28%8%29%25%9%1
360
11th to 20thNight
18%11%37%28%6%1
354
21st to endNight
16%11%31%32%9%
389
June1st to 10thNight
23%8%26%36%7%
360
11th to 20thNight
18%9%29%31%12%1
360
21st to endNight
18%9%30%31%11%
360
July1st to 10thNight
11%5%30%43%9%1
350
11th to 20thNight
13%5%28%35%17%1
348
21st to endNight
14%6%31%37%10%1
394
August1st to 10thNight
16%5%23%39%16%1
385
11th to 20thNight
19%420%35%20%2
583
21st to endNight
14%7%30%32%16%1
792
September1st to 10thNight
19%8%27%30%16%1
720
11th to 20thNight
19%7%21%32%17%4
720
21st to endNight
19%11%25%27%17%2
720
October1st to 10thNight
27%12%27%19%13%2
720
11th to 20thNight
27%13%27%24%8%1
720
21st to endNight
35%12%28%21%4
792
November1st to 10thNight
29%15%29%20%7%1
740
11th to 20thNight
28%18%27%16%9%1
760
21st to endNight
34%19%24%17%6%
760
December1st to 10thNight
38%18%25%16%3
760
11th to 20thNight
32%20%30%14%31
760
21st to endNight
37%16%27%16%3
836
Page generated at 2024-01-28 05:26:32. Version ac07

Copyright 2024 Attilla Danko


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